In computer science, a virtual machine (VM) is a portion of software that, when executed on appropriate hardware, creates an environment allowing the virtualization of an actual physical computer system. Each VM may function as a self-contained platform, running its own operating system (OS) and software applications (processes). Typically, a virtual machine monitor (VMM) manages allocation and virtualization of computer resources and performs context switching, as may be necessary, to cycle between various VMs.
A host machine (e.g., computer or server) is typically enabled to simultaneously run multiple VMs, where each VM may be used by a local or remote client. The host machine allocates a certain amount of the host's resources to each of the VMs. Each VM is then able to use the allocated resources to execute applications, including operating systems known as guest operating systems. The VMM virtualizes the underlying hardware of the host machine or emulates hardware devices, making the use of the VM transparent to the guest operating system or the remote client that uses the VM.
In a virtualization systems, it is important to schedule (also referred to as selection or placement) VMs on the “best host” for the job. There are various considerations that play into a selection algorithm for selecting the “best host”. The selection algorithm is relevant for both placement of the VM at launch time, and for live migration of the VM to another host. Live migration may be due to user request, or due to a trigger by a load balancing process causing the need to select a new host for the VM.
VMs may run an operating system (OS) and processes with the same portions of the memory image as other VMs. In such a case, a host process may be used to optimize memory utilization on the host, by referencing identical memory pages from multiple VMs to the same single memory page. This is known as memory sharing. Memory sharing reduces the amount of memory required in the host when running VMs with shared memory.
Thus, a virtualization system would benefit from a host selection algorithm optimized to schedule VMs with high memory sharing on the same host.